Separation by implanted oxygen silicon wafers have been investigated with cross‐sectional transmission electron microscopy after annealing at 1200 °C in argon as well as in hydrogen. It is observed that the buried oxide has experienced little change in the thickness, which is ascribed to the low hydrogen solubility in the crystal (about 3×1015cm−3) and provides a natural mark to measure the thickness variation of the top silicon directly. The evaporation rate as determined in this method is less than 0.1 nm/min with an accuracy of ±0.1 nm/min, at least two orders of magnitude lower than reported in previous investigations. ©1995 American Institute of Physics.